Editorial Type:
Article Category: Research Article
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Online Publication Date: 04 Mar 2014

Canine retraction and anchorage loss: Self-ligating versus conventional brackets in a randomized split-mouth study

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Page Range: 846 – 852
DOI: 10.2319/100813-743.1
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ABSTRACT

Objective:

To evaluate the velocity of canine retraction, anchorage loss and changes on canine and first molar inclinations using self-ligating and conventional brackets.

Materials and Methods:

Twenty-five adults with Class I malocclusion and a treatment plan involving extractions of four first premolars were selected for this randomized split-mouth control trial. Patients had either conventional or self-ligating brackets bonded to maxillary canines randomly. Retraction was accomplished using 100-g nickel-titanium closed coil springs, which were reactivated every 4 weeks. Oblique radiographs were taken before and after canine retraction was completed, and the cephalograms were superimposed on stable structures of the maxilla. Cephalometric points were digitized twice by a blinded operator for error control, and the following landmarks were collected: canine cusp and apex horizontal changes, molar cusp and apex horizontal changes, and angulation changes in canines and molars. The blinded data, which were normally distributed, were analyzed through paired t-tests for group differences.

Results:

No differences were found between the two groups for all variables tested.

Conclusions:

Both brackets showed the same velocity of canine retraction and loss of anteroposterior anchorage of the molars. No changes were found between brackets regarding the inclination of canines and first molars.

Keywords: Canine retraction; Anchorage; Self-ligating brackets; Tooth movement rate

INTRODUCTION

Several in vitro studies have reported lower friction levels when self-ligating brackets (SLB) were compared to conventional brackets (CB).1–8 It would be logical, therefore, to assume that spaces could be closed faster when SLB are used, since it is known that friction could influence movement rates. Controversially, recent systematic reviews failed to report the superiority of SLB over CB when tooth movement velocity was assessed,9,10 challenging what logically would make sense. Additionally, several randomized clinical trials using SLB have been conducted,10–19 and it has been suggested20 that more clinical trials are needed to assess tooth velocity during space closure when SLB and CB are compared.

Canine retraction is probably the most common clinical situation where sliding mechanics is used to move a tooth over a relatively large distance. Therefore, it would be interesting to evaluate the “superiority” of one bracket over another regarding friction, but to date only three studies have compared the velocity of canine retraction using SLB and CB,11,14,16 and their results are controversial. Two14,16 have failed to find differences between those brackets, while the remaining11 favored CBs. Even though the design of the latter study11 allowed a more complete evaluation since full canine retraction was evaluated, measurements were taken directly in the mouth and rounded to the half millimeter, which could explain the small differences found. Additionally, no information on tipping was collected, and differences in tipping could explain the differences found.

Another claim regarding SLB, involves the belief that they would allow less anteroposterior (AP) anchorage loss of the molars during space closure. This idea comes from the theory that less friction would allow lighter forces to retract anterior teeth and, therefore, suboptimal forces would be applied to the posterior teeth.21 Three clinical trials have examined this hypothesis,14,22,23 but only one evaluated the loss of AP anchorage during canine retraction and only over a period of 12 weeks.14 Three months may not be enough to detect differences between brackets, and a longer period of evaluation could be desirable.

Thus, the aim of this study was to assess possible differences in canine retraction velocity, as well as changes in tipping, and the amount of AP anchorage loss during maxillary canine retraction, using CB and SLB.

MATERIALS AND METHODS

Sample size was calculated using the PS: Power and Sample Size Calculation software, version 3.0.43 (Vanderbilt University, Nashville, TN) available at http://www.mc.vanderbilt.edu/prevmed/ps/index.htm. Based on estimated differences between groups of 0.2 mm per month and on a standard deviation of 0.3 mm,14 a sample size of 25 patients per group was calculated for an α of .05 and a power of 90%.

Twenty-five Class I biprotrusive adult patients requiring four first premolar extractions, with arch length discrepancy below 4 mm, were selected for the study out of 52 patients who sought treatment at the Araraquara School of Dentistry (Table 1). Three patients did not attend the initial screening, and 24 were excluded due to tooth loss or arch length discrepancy above 4 mm. The university's institutional review board approved this research, and the patients signed an informed consent agreeing to participate in the study. Patients were allocated into five groups, with five patients in each; a block randomization was done to determine which canine, right or left, would have the SLB or CB bonded. Figure 1 depicts the flow diagram of the progress through the phases of a parallel-randomized trial of two groups.

Figure 1. Flow diagram of the progress through the phases of study.Figure 1. Flow diagram of the progress through the phases of study.Figure 1. Flow diagram of the progress through the phases of study.
Figure 1. Flow diagram of the progress through the phases of study.

Citation: The Angle Orthodontist 84, 5; 10.2319/100813-743.1

Table 1. Sample Characteristics
Table 1.

Conventional 0.022-inch straight-wire brackets (Ovation, GAC, Bohemia, NY) were bonded to the upper second premolars, and incisors and tubes were soldered to bands of the first and second molars. Through block randomization, one maxillary canine was bonded with a 0.022-inch SLB (In-Ovation R, GAC), while the other received a 0.022-inch CB (Ovation, GAC). Leveling and aligning was conducted conventionally until a 0.020-inch stainless steel (SS) wire could be passively inserted into the brackets.

Retraction was undertaken on 0.020-inch SS archwires with tight fit omega loops tied to the first molars. Second premolars and first and second molars were tied together with ligature wire, which were also used to tie the canines CB to the archwire. No auxiliary devices such as transpalatal arches, headgear, or elastics were used. Nickel-titanium closed coil springs (CCS) of 100 g (GAC) were activated for 17 mm and secured from the hooks of first molars to the hooks of the canine brackets with ligature wires. The CCS were adjusted to the same activation every 4 to 5 weeks.

Oblique cephalometric radiographs at 45° were taken from right and left sides (T1) at 7 to 14 days before the extractions and after total canine retraction (T2). All 25 patients reached the T2 phase without bracket debonds or CCS breakage.

The tracings were hand drawn with a 0.3-mm mechanical pencil on tracing paper by one operator. Two horizontal reference points were marked (Figure 2; Table 2) over the functional occlusal plane traced in T1 in order to determine the horizontal reference line (HRL). Point 1 was located in the anterior region of the tracing, and point 2 was in the posterior region of the tracing. A third point was marked above the orbit contour and posterior to the tracing to determine a vertical reference line (VRL) perpendicular to HRL.

Figure 2. Tracing with anatomic cephalometric landmarks and reference points as described in Table 2.Figure 2. Tracing with anatomic cephalometric landmarks and reference points as described in Table 2.Figure 2. Tracing with anatomic cephalometric landmarks and reference points as described in Table 2.
Figure 2. Tracing with anatomic cephalometric landmarks and reference points as described in Table 2.

Citation: The Angle Orthodontist 84, 5; 10.2319/100813-743.1

Table 2. Description of Cephalometric Landmarks and Reference Points Identified on the 45° Lateral Tracing
Table 2.

Four points were marked on each of the right and left radiographs: (MC) and (MA) and (CC) and (CA) (Table 2). All anatomic landmarks and points were digitized on the software DentoFacial Planner Plus (DFP version 2.0, Toronto, ON, Canada).

T1 and T2 tracings were superimposed using the best fit of maxillary bone structures, following the internal anterior maxillary cortex at the contralateral canine, posterior contour of the infrazygomatic crest, nasal cavity floor, and anterior lower orbit bone contour (Figure 3).24 The three reference points were then transferred from T1 to T2.

Figure 3. Partial superimposition of T1 and T2. The superimposed stable structures of the maxillary bone complex, the three reference points, and the lines used to measure the horizontal displacement of the teeth can be seen.Figure 3. Partial superimposition of T1 and T2. The superimposed stable structures of the maxillary bone complex, the three reference points, and the lines used to measure the horizontal displacement of the teeth can be seen.Figure 3. Partial superimposition of T1 and T2. The superimposed stable structures of the maxillary bone complex, the three reference points, and the lines used to measure the horizontal displacement of the teeth can be seen.
Figure 3. Partial superimposition of T1 and T2. The superimposed stable structures of the maxillary bone complex, the three reference points, and the lines used to measure the horizontal displacement of the teeth can be seen.

Citation: The Angle Orthodontist 84, 5; 10.2319/100813-743.1

The distances, parallel to HRL, from the points CC, CA, MC, and CA to VRL were transferred to a Microsoft Excel, (Microsoft Office Excel 9.0, Redmond, WA, USA) spreadsheet where T2 values were subtracted from T1 values. These calculations allowed the measurement of the total amount of canine retraction, at the cusp and at the apex, and measurement of its velocity, by dividing the total amount of retraction by the number of days for total space closure on each side. The amount of AP loss of anchorage of the molar cusps and apices was also calculated similarly.

The angle formed by the tooth long axis, determined by the cusp and apex points and HRL, was measured in T1 and T2. This allowed an assessment of the changes that occurred on tooth angulations also by subtracting T2 from T1 (Table 2).

A single blinded examiner traced all radiographs and superimposed and digitized them. Each digitalization was repeated after 30 days for method error analysis and also for the use of average measurements. Error analysis was done though a paired t-test, which evaluated systematic error and was complemented by the Dahlberg formula, which evaluated random error. No significant differences were found on the paired t-test between the first and second digitalization times, which confirmed the absence of systematic error, while the Dahlberg formula showed random errors ranging from 0.21 mm to 0.36 mm and from 0.77° to 1.28°.

Statistics were conducted with the software Statistical Package for Social Sciences, SPSS, version 16.0 (SPSS, Chicago, IL, USA). All variables were normally distributed according to kurtosis and skewness standard error comparisons; thus, paired t-tests were used to detect differences between groups.

RESULTS

The canine crowns were retracted 0.71 and 0.72 mm per month, and apices were retracted 0.22 and 0.24 mm per month, for SLB and CB, respectively (Tables 3 and 4). Total retraction of the canine crowns was 6.92 and 6.97 mm, while the total retraction of the apices was 2.24 and 2.43 mm, for the SLB and CB groups, respectively. Molars were protracted 1.28 mm in the SLB group and 1.24 mm in the CB group, while their apices were protracted 0.60 mm in the SLB group and 0.52 mm in the CB group. Total time for space closure took 10.86 months and 10.70 months for the SLB and CB groups, respectively. No significant differences were found between the groups for any of the variables tested (Tables 5 and 6).

Table 3. Means, Standard Deviations, and Significance of the Comparisons Between Groupsa for the Variables Tested Monthly (28 Days)
Table 3.
Table 4. Average Changesa and Standard Deviations for Canine and Molar Inclinations in Both Groupsb and Significance of the Differences Found
Table 4.

DISCUSSION

No differences were found between SLB and CB in the velocity of canine retraction. To date, only three clinical studies (Table 7) have compared the velocity of retraction of maxillary canines with SLB and CB,11,14,16 and even though different methods were employed, the current results were similar to two of them.14,16 The remaining study11 identified a higher velocity of retraction for CB and hypothesized that the differences found were due to differences in the widths of the brackets. We tried to control that variable by using SLB and CB from the same manufacturer and of very similar widths (2.9 mm and 3.4 mm, respectively), and even then no differences were found. However, there was a wide range of responses among patients, as well as different responses within patients (Figure 4).25–27 On the other hand, we found a lower velocity of canine retraction compared to other studies.11,14,16 Two factors may have contributed to that finding: the adult sample, which may show slower velocity of movement,28,29 and the force level (100 g), which could have been suboptimal for the friction developed by a high-diameter wire (0.020-inch).11,14,26

Figure 4. Canine retraction velocities per month with SLB and CB in each patient of the sample.Figure 4. Canine retraction velocities per month with SLB and CB in each patient of the sample.Figure 4. Canine retraction velocities per month with SLB and CB in each patient of the sample.
Figure 4. Canine retraction velocities per month with SLB and CB in each patient of the sample.

Citation: The Angle Orthodontist 84, 5; 10.2319/100813-743.1

Table 5. Mean Differences, Standard Deviations, Standard Errors of the Mean, and 95% Confidence Intervals of the Difference Between SLB and CBa
Table 5.
Table 6. Mean Differences, Standard Deviations, Standard Errors of the Mean, and 95% Confidence Interval of the Difference Between SLB and CB for Canine and Molar Inclinationsa
Table 6.
Table 7. Data From Articles on Maxillary Canine Retraction Velocity Comparing Self-Ligating Brackets (SLB) and Conventional Brackets (CB)
Table 7.

There was no difference in the loss of AP anchorage of the molar crowns between SLB and CV. Compared to total retraction, we found a retraction to loss-of-anchorage ratio of 5.4∶1 for the SLB and 5.6∶1 for the CB. Only one other study14 has compared the loss of anchorage during canine retraction between SLB and CB, and it also reported no differences between bracket types. The ratio of retraction to loss of anchorage was slightly lower than ours (4∶1 and 4.3∶1 for SLB and CB, respectively), but the time of evaluation in that study was only 8 weeks. Two other papers that have compared SLB and CB found a greater anchorage loss compared to our study. One23 reported ratios of 1.3∶1 and 1∶1 for SLB and CB, respectively, while the other22 found low ratios of 0.19∶1 for both SLB and CB, but those papers evaluated total space closure as opposed to canine retraction only.

There was no difference in the tipping of the canines and molars when the bracket types were compared. Only one16 of the articles comparing SLB and CB during canine retraction evaluated this variable and also found no difference. Tipping should always be assessed when studying space closure because differences might confound the apparent velocity of movements since tipping is generally associated with faster movement than translation.30–32 In order to control this variable in the current study, all teeth were aligned and leveled up to a .020-inch SS wire before extractions took place. This provided a standardization of the initial position of the teeth before canine retraction was initiated and thus avoided differences in the initial position of the canines which could have influenced the rate of movement.33 The use of a round SS wire with low second order clearance (0.020-inch wire in a 0.022-inch slot) combined with a relatively low force (100 g) was chosen to provide maximum canine translation34 and less crown tipping. After total retraction, canine crowns moved distally more than the apices did, resulting in 10° to 11° of change in the canine long axes. The same occurred with the molars, but the change was approximately 2°. This difference in tipping between canines and molars can be explained by the smaller second order clearance of molar tubes compared to canine brackets due to the difference in width.

An a priori sample size calculation was done for this research. A sample size of 25 was calculated using a significance of 0.05, a power of 90%, a difference 0.2 mm per month (considered by the authors to be relevant), and a standard deviation of 0.3 mm per month, which was selected from the literature.14 The post hoc test of our data, however, showed a power of only 5.2% to rule out a type II error with the small difference found (0.01 mm per month). Even though we had a small sample size to rule out differences of 0.01 mm per month, we believe that from a clinical standpoint those differences would be clinically insignificant.

The results of this study demonstrated that bracket type does not influence velocity of tooth movement, but individual variation might. If the patients tested were divided into two groups based on the movement rate, for comparison of the fast (above average) and slow (below average) “movers,” a significant difference of 0.304 mm per month in the rate of canine retraction would be observed (Table 8). This difference is much higher than the nonsignificant difference found when the bracket type groups were compared or, assuming that a type II error occurred, 30 times higher when compared to the difference of 0.01 mm per month between SLB and CB. In another words, the rate of canine movement seems to be influenced by individual biological responses of patients, rather than by bracket type. Without management of the biological responses that occur after a force is applied to a tooth, it would be very difficult to observe a faster velocity of tooth movement in the future.35–39

Table 8. Maxillary Canine Displacement Velocities/Month Demonstrated by Patients Divided in Two Groups Based on the “Speed of Movement”
Table 8.

Based on the results of this study, in a given population, no significant difference in the rate of tooth movement is likely to be found between groups with different bracket types. On the other hand, it is not possible to predict accurately the time required for tooth retraction in individual patients due to the great amount of variability observed.

CONCLUSIONS

When comparing SLB and CB:

  • retraction velocities of the maxillary canines were similar;

  • anchorage loss during maxillary canine retraction was similar; and

  • inclination changes on maxillary canines and molars were similar.

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Copyright: © 2014 by The EH Angle Education and Research Foundation, Inc.
Figure 1.
Figure 1.

Flow diagram of the progress through the phases of study.

Figure 2.
Figure 2.

Tracing with anatomic cephalometric landmarks and reference points as described in Table 2.

Figure 3.
Figure 3.

Partial superimposition of T1 and T2. The superimposed stable structures of the maxillary bone complex, the three reference points, and the lines used to measure the horizontal displacement of the teeth can be seen.

Figure 4.
Figure 4.

Canine retraction velocities per month with SLB and CB in each patient of the sample.

Contributor Notes

Corresponding author: Dr Luiz Gonzaga Gandini Júnior, Av Casemiro Perez, 560, Vila Harmonia, Araraquara, São Paulo, CEP 14802-600, Brazil (e-mail: luizgandini@uol.com.br)
Received: 01 Oct 2013
Accepted: 01 Jan 2014
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